1. Field of the Invention
This invention relates to the fabrication of electronic components, and more particularly, to a low cost, highly productive process of fabricating multilayer ceramic components with interlayer conductive-filled via holes.
2. Description of the Related Art
A variety of commercially available active electronic components have included multiple ceramic layers with interconnected circuits formed in each layer. In addition, hybrid circuits, integrated circuit packages, and transformers have been commercially available which have included multiple ceramic layers. Passive devices such as capacitors and inductors have also been commercially available which have included multiple ceramic layers.
There are several conventional methods of manufacturing multilayer ceramic components. Each requires the formation of thin layers of ceramic having the desired electrical properties and the delineation of a pattern or a matrix of patterns of conductive circuit pathways or electrodes between the layers of ceramic. Interconnection between the ceramic layers, when required, is provided by placing via holes in the layers at the desired locations and filling such via holes with conductive materials which contact the printed conductive circuit pathways to allow electron flow between the ceramic layers.
Typically, multilayer components consist of a bottom insulating layer of ceramic, one or more layers of ceramic with conductive circuitry applied to a top insulating layer of ceramic. Conductive circuitry may also be applied to the top surface.
There are several steps common to conventional methods of manufacturing multilayer ceramic components. One major step is to form a ceramic base. It is desirable to start with a uniform mix of ceramic constituents which is free from impurities or bubbles caused by trapped gases. Another major step is the deposition of the conductive electrode circuit pathway pattern onto the ceramic base. This requires compatibility between the electrode chemistry and the ceramic composition chemistry. Where screen printing techniques are used, compatibility is typically achieved by the proper selection of the electrode ink used to print the circuit pattern. It is necessary to dry the printed patterns without contamination, smearing or distortion.
For components requiring interlayer connection another major step involves forming a via to provide electrically connectivity of the electrode to other electrodes in additional circuit layers. Via holes must be correctly positioned to match the circuit pathways. They must have a fairly consistent diameter and must be free from debris internally or externally. The ceramic material around the via holes must be free from stress that might induce distortion or cracking. Once the via through-holes are formed they must be filled with conductive material.
Still another important step involves the stacking of layers of ceramic material imprinted with the electrode circuit pattern. During this step, the electrode patterns must be vertically aligned without being distorted. The stacking must be done in a delicate manner to prevent damage to the ceramic material. Air must not be trapped between the alternating layers of ceramic material and electrodes. A top layer of ceramic equivalent to the ceramic base may also be required in the design of some components.
Generally, monolithic multilayer electronic components may be built according to either of two types of processes. One process is known as a "tape process" whereby ceramic slurry is cast into a thin film and dried to form a ceramic tape. Multiple sheets of the ceramic tape may be stacked to form the ceramic base and top layers. Each conductive circuit pathway layer is printed on a ceramic tape surface and then a plurality of such tape surfaces along with the base and top layers, are registered, stacked and laminated together.
In a tape process, a ceramic slurry is cast onto a thin film of tape that is carried on a substrate, such as steel belt or polymer film. The dried tape is stripped from the substrate. A printed circuit path is printed on the tape, and then a monolithic circuit is formed by stacking several such printed tape circuits one on top of another to form a multilayer construction. The tape layers are laminated together by applying heat and pressure. To electrically interconnect the layers vias are formed by making holes in each tape layer. The holes are typically formed with a mechanical punch or a laser prior to printing the electrode and prior to stacking. The holes must then be filled with some conductive material to form a conductive path between adjoining layers.
In a typical tape process for manufacturing multilayer components with interconnecting conductive layers discrete equipment and discrete operations are required for casting, stripping, via formation, printing, stacking and laminating.
In a "wet process", ceramic slurry is cast and dried on a rigid substrate to form the ceramic base. A printed circuit made of conductive material is printed on the base and dried. The next layer of slurry is cast on top of the first layer, dried and printed with an electrical circuit. This process is repeated until the desired number of layers have been formed at which point the top ceramic layers are cast. An advantage of this process is that only two operations are required, namely casting and printing and these operations are easily combined and automated in a single machine resulting in reduced labor costs, increased automation and productivity. However, when conductive vias must be placed between adjoining circuit layers, the wet process has been of limited value. Prior art techniques have resorted to screen printing of the ceramic slurry as opposed to casting of the slurry so that suitable openings in the ceramic layer could be formed to serve as vias. These techniques require numerous screen printers and have resulted in a process which is much slower and much more difficult to control the quality of the vias, thereby negating the inherent advantages of a wet process.
It would clearly be an advancement of the art to gain the productivity advantage of the wet process in the manufacturing of multilayer components which require conductive vias between layers.